Abstract
Background: This study examined the motor unit (MU) control strategies for non-fatiguing isometric elbow flexion tasks at 40% and 70% maximal voluntary isometric contraction. Methods: Nineteen healthy individuals performed two submaximal tasks with similar torque levels: contracting against an immovable object (force task), and maintaining the elbow joint angle against an external load (position task). Surface electromyographic (EMG) signals were collected from the agonist and antagonist muscles. The signals from the agonist were decomposed into individual action potential trains. The linear regression analysis was used to examine the MU recruitment threshold (RT) versus mean firing rates (MFR), and RT versus derecruitment threshold (DT) relationships. Results: Both agonist and antagonist muscles’ EMG amplitudes did not differ between two tasks. The linear slopes of the MU RT versus MFR and RT versus DT relationships during the position task were more negative (p = 0.010) and more positive (p = 0.023), respectively, when compared to the force task. Conclusions: To produce a similar force output, the position task may rely less on the recruitment of relatively high-threshold MUs. Additionally, as the force output decreases, MUs tend to derecruit at a higher force level during the position task.
Highlights
Skeletal muscle contractions consist of dynamic and static muscle actions
The results showed that overall motor unit (MU) behaviors were similar regarding MU mean firing rates (MFR) and recruitment threshold (RT) between two tasks
The main findings of this study were: (1) The surface EMG amplitude did not differ between two tasks at both intensities for both agonist and antagonist muscles; and (2) The linear slope coefficients of the MU RT vs. MFR as well as the RT vs. derecruitment threshold (DT) relationships were significantly different between the two tasks
Summary
Skeletal muscle contractions consist of dynamic (e.g., concentric and eccentric contractions) and static muscle actions (e.g., isometric contraction). Based on the load type applied on the muscle, the isometric muscle actions can occur during two different tasks: contracting muscles against an immovable object (force task), and holding still against an external load (position task). A variety of factors such as the modulations of the motor unit (MU) activities (discharging and recruitment) [8] can contribute to the task-related differences in skeletal muscle fatigue [9]. This study examined the motor unit (MU) control strategies for non-fatiguing isometric elbow flexion tasks at 40% and 70% maximal voluntary isometric contraction. Methods: Nineteen healthy individuals performed two submaximal tasks with similar torque levels: contracting against an immovable object (force task), and maintaining the elbow joint angle against an external load (position task).
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